The present invention relates to liquid crystal display devices and more particularly to color liquid crystal display devices in which a liquid crystal element of the twisted nematic type (hereinafter referred to as TN type) and a color polarizer plate are combined so as to provide multicolor display.
Many conventional liquid crystal multicolor display devices are of the system in which a liquid crystal display element which functions as a light shutter is used in combination with multicolor filters or polarizer plates. Particularly, the system mentioned above includes a system in which a black dichroic matter is added to a liquid crystal to be used as a light shutter element of the guest-host type, a system in which a TN shutter element constituted by two, upper and lower, polarizer plates of TN type arranged with their polarization axes made in the same direction so as to be black when no voltage is applied while allow light to pass therethrough upon application of a voltage is combined with color filters, and so on.
However, these systems have disadvantages. That is, in the former system using a black guest-host type, the change characteristic of light transmission factor is gentle in the vicinity of a threshold value due to the applied voltage, so that it is difficult to drive the device in the time-divisional mode but the device can be driven statically. Further, in order to make the contrast high between the case where a voltage is applied to the device and the case where no voltage applied to the same, the quantity of addition of the black material must be increased, and, if so, the quantity of light transmission upon the application of voltage is reduced to make the display per se dark. In some cases, alternatively, the device is arranged to have a two-layered cell construction. This arrangement may provide, however, a significant factor of increase in cost.
In the latter case, on the other hand, although the threshold characteristic and the contrast are improved at a large extent in comparison with the guest-host system, the display has dependency on visual angle and good contrast can be obtained when viewed only in the specific direction. The dependency on visual angle is an essential characteristic of the TN system and it is possible to intensify the contrast and widen the effective visual angle by reducing the product .DELTA.n.multidot.d between the refractive index anisotropy .DELTA.n and the cell gap thickness d. If the value of this product .DELTA.n.multidot.d is reduced too much, however, the light transmissive efficiency of the TN liquid crystal element is lowered and interference color may occur upon application of no voltage to thereby excessively deteriorate the quality of display. Accordingly, the lower limit value of .DELTA.n.multidot.d is the order of but no more than 1.2(m.mu.m).
The display mode of the TN type liquid crystal element used in ordinary electronic calculators or wrist watches is a positive display system in which the display color is transparent upon application of no voltage while black upon application of a voltage. Accordingly, if the value .DELTA.n.multidot.d is small, interference color is generated at transparent portions. Since a silver or white reflection plate is used, generally, the interference color is not so conspicuous, and if the cell gap is uniform, the color tone of the interference color is also uniform all over the area so that, in many cases, the interference color does not cause inconvenience in practical use even if it is generated a little. Recently, conjointly with the fact that variations in the cell gap have been decreased with the advance of manufacturing technique, the value .DELTA.n.multidot.d has a tendency to be designed to have a small value such as 1.0 or 0.8. In the case where the TN type liquid crystal element is used as a light shutter for multicolor display, however, the displaying color is black upon application of no voltage, and, therefore, interference color is very conspicuous if generated. Accordingly, in this case, the value .DELTA.n.multidot.d is compelled to be made large at the sacrifice of the visual angle as well as the contrast, in order to prevent the interference color form occuring.